1. Field of the Invention
This invention pertains in general to an improved method for removing sulfur and nitrogen oxides from flue gases. In particular, it relates to a method for converting flue-gas sulfur and nitrogen oxides to commercial-grade ammonium sulfate during the process of regeneration of scrubbing liquors.
2. Description of the Prior Art
Fossil-fuel combustion produces sulfur and nitrogen oxide pollutants that must be substantially removed from flue gases prior to their discharge to the atmosphere. This requirement introduces a material expense to plant operations and is particularly significant in the case of coal-operated power plants. Therefore, much effort has been devoted to the optimization of flue-gas scrubbing processes and to the development of alternative solutions to the problem of sulfur and nitrogen oxides removal.
The removal of these compounds from flue gases is typically achieved by well known processes wherein the flue gases are contacted with a wet scrubbing slurry containing reactants that cause the sulfur and nitrogen oxides to precipitate out of the gas stream. Calcium hydroxide, sometimes also containing parts of magnesium hydroxide, and ferrous ethylenediaminetetraacetic acid (Fe.sup.II.EDTA) are known to be very effective for the removal of sulfur and nitrogen oxides. Accordingly, various similar processes utilize aqueous solutions of lime and/or limestone (CaO and CaCO.sub.3, respectively) as the scrubbing liquor and Fe.sup.II.EDTA is added as a promoter for the removal of nitrogen oxides. See, for example, U.S. Pat. No. 5,200,160, which describes a method for optimizing the utilization of Fe.sup.II.EDTA in such a process.
A characteristic aspect of such CaO/Fe.sup.II.EDTA wet scrubbing processes for the simultaneous removal of SOx and NOx is the accumulation, in addition to solid calcium sulfite (CaSO.sub.3. 1/2H.sub.2 O), of nitrogen and sulfur compounds (N-S compounds) in the scrubbing liquor (FGL). These compounds assume a variety of compositions, mainly consisting of hydroxylamine disulfonic acid (HADS), hydroxylamine monosulfonic acid (HAMS), amine trisulfonic acid (ATS), amine disulfonic acid (ADS), sulfamic acid (SA), and hydroxylamine (HA). Although not certain, the major reaction pathways of N-S compounds in a wet scrubbing solution have probably been shown by Chang, S.G., in "Technical Analyses of a Wet Process for Flue Gas Simultaneous Desulfurization and Denitrification," in Fossil Fuels Utilization: Environmental Concerns, Markuszwesk, R., Blaustem, B. D., Eds., ACS Symposium Series, No. 319, 159-175, 1986, and are illustrated in FIG. 1. These N-S compounds are preferably removed from the scrubbing liquor before it is either discarded or recycled.
Since most scrubbing processes utilize lime/limestone in the scrubbing liquor, a precipitate of calcium sulfate and sulfite (CaSO.sub.4 /CaSO.sub.3) is produced in the liquor during scrubbing. In the case of CaO/MgO/Fe.sup.II.EDTA scrubbers, we found that what was previously considered to be simply a CaSO.sub.4 /CaSO.sub.3 mixture in fact contains a significant amount of N-S compounds. These precipitates are usually dumped in landfills; therefore, it would be desirable to remove them prior to dumping.
Some processes for achieving that end treat the scrubbing liquor effluent from the flue-gas scrubber with sulfuric acid in order to decompose the N-S compounds by hydrolysis. See, for example, the Chisso Engineering Process and the Mitsubishi Heavy Industry Process. Because sulfuric acid is added to the scrubbing liquor (which is more than 95 weight-percent water and, therefore, requires large amounts of acid), this type of process has not been economical. Another known process (e.g., the Asahi Chemical Process) selectively separates the N-S compounds from the scrubbing liquor and then decomposes them to yield N.sub.2. Because no marketable byproduct is obtained from the decomposition of the N-S compounds, this type of process is also considered less than economical. Yet another process (upon which the invention of U.S. Pat. No. 5,200,160 is based) recycles a portion of the aqueous scrubbing slurry discharged from the liquid scrubbing unit and passes the remainder through a thickener where a clarified liquor and a thickener underflow are produced. The clarified liquor is returned to the scrubbing unit, while the underflow is further treated to remove sulfur and nitrogen components in a separate operation from the scrubbing process.
In an effort to improve these processes, we have previously focused our work on experiments based on the use of K.sub.2 SO.sub.4 to isolate the N-S components of spent flue-gas scrubbing liquor. The N-S compounds were precipitated as potassium salts by a metathetical reaction with K.sub.2 SO.sub.4. The solid potassium salts were then converted by hydrolysis in a sulfuric acid environment to ammonium and potassium sulfates that constitute a salable product as a mixed fertilizer. This investigation revealed that more than 60% of the added potassium remained in the scrubbing liquor after precipitation of the N-S compounds as potassium salts. Therefore, the loss of dissolved potassium in the recycled mother liquor to the scrubbing unit, probably as entrainment on the gypsum precipitate, is a serious detriment to the economic viability of this regeneration process.
This invention is directed at a process that avoids the environmental and economic problems, of prior-art methods; at the same time, it enhances the economic viability of scrubbing-liquor regeneration by directly producing commercial-grade ammonium sulfate and Ca/Mg sulfates from the N-S compounds removed from flue gases. The invention incorporates the recovery of nitrogen and sulfur with the scrubbing-liquor regeneration process itself, rather than viewing such recovery only as a separate, potentially-useful downstream option.